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Purpose: To investigate the effects of high-intensity interval training (HIT) on physical fitness and cardiometabolic health in youth with physical disabilities.

Methods: For this quasi-experimental study 70 participants were recruited from schools for special education and divided into runners and users of wheelchairs. HIT was performed for 8 weeks, twice a week, containing 30 seconds all-out exercises.

Results: Exercise adherence was 84.5%. Following HIT, there were improvements in anaerobic performance, agility, aerobic performance, and systolic and diastolic blood pressure. There were no changes in peak oxygen uptake (

INTRODUCTION

Given the fact that youth (ie, children [6-12 years] and adolescents [13-19 years]) with physical disabilities have lower physical fitness levels compared with their peers without disabilities,1 they are thought to be at increased risk for cardiometabolic diseases.2 Several studies have reported positive effects on physical fitness of more traditional forms of exercise training in youth with cerebral palsy (CP) and spina bifida,3,4 the most common physical disabilities in youth. However, high-intensity interval training (HIT) is a more effective and time-efficient form of training in adults with cardiometabolic diseases.5 HIT involves alternating short bursts of high-intensity exercise followed by a period of low-intensity exercise or rest.6 Despite large differences in training volume (ie, 90% lower in HIT) and time commitment (ie, 67% lower in HIT), HIT provides similar or greater benefits in improving physical fitness and reducing cardiometabolic risk factors when compared with moderate-intensity continuous training (MICT) in both adults who are healthy and adults with cardiometabolic diseases.5,7

Following HIT, positive effects on performance-related fitness, such as a reduction in exercise time, improved sprint performance or agility, can be expected.6,7 Performance-related fitness is the combined result of coordinated exertion with a variety of physiological functions, is related to performance in daily life activities, and can be divided into aerobic and anaerobic performance.8 Cardiorespiratory fitness, expressed as peak oxygen uptake (

O2peak), almost doubled after HIT as compared with MICT in a meta-analysis among adults with lifestyle-induced cardiometabolic disease.5 In addition to the physical benefits, HIT also improves cardiometabolic health, as it improves both cardiovascular (ie, blood pressure and arterial stiffness) and metabolic risk factors (ie, body composition, lipid profile, and fasting glucose) in both healthy and diseased adults.5,9,10

Daily physical activity in youth most often consists of brief, intermittent bouts of intense movement, similar to HIT.11 Since physiological adaptations after training are task specific in general,8 HIT might be more relevant to the activity patterns during childhood and adolescence as compared with MICT.12 This holds true for youth without motor problems. However, it is unknown whether intermittent bouts of intense exercise have the same effect on youth with physical disabilities. In addition, since performance-related fitness is partly dependent on motor function,13 it is unknown how HIT affects children and adolescents with different levels of mobility.

The higher intensities of HIT cause greater disturbances to the physiological system, resulting in enhanced metabolism greater or similar to MICT.6 Several studies have shown short-term (ie, ≤12 weeks) positive effects of HIT on exercise performance,14

O2peak,15 body mass index,15 blood pressure,15 body composition,14,15 and fasting glucose15 in children and adolescents who are overweight. A recent study supported that

O2peak increased and body composition remained similar after 24 HIT sessions in children with CP.16 However, little is known about the effect of HIT on fitness and health in children and adolescents with disabilities. The primary aim of the current study was to investigate the effects of 8 weeks of HIT on anaerobic performance in youth with physical disabilities including different levels of mobility. The effects on agility, aerobic performance,

METHODS

This study was a quasi-experimental study with pre- and postmeasurements. All children and adolescents performed HIT for 8 weeks, twice a week, for 30 minutes. Outcome measures were assessed at baseline (T0) and immediately after HIT (T1) by the same trained researcher across all schools. The current study was part of the Sport-2-Stay-Fit study (Dutch Trial Register #NTR4698), where HIT was followed by a school-based sports program comparing an intervention with a control group. HIT was therefore performed by all participants as an initial start-up to determine their fitness level and to get familiar with exercise. The full protocol is described elsewhere.17 Both ethics approval and administrative site approvals were granted by the Medical Ethical Committee of University Medical Center Utrecht in the Netherlands (#14-118).

Participants

Children and adolescents, all able to walk and those propelling a manual wheelchair, were recruited from 4 schools for special education in the Netherlands. These schools have similar learning objectives as other schools, but the children receive additional attention and support. Rehabilitation medicine is provided within school hours by rehabilitation professionals. Of all youth in primary or secondary education, 1.9% (26 500) go to these schools for special education in the Netherlands.18,19Children and adolescents were screened for eligibility by a physical therapist, physical educator, or physician. Inclusion criteria for the Sport-2-Stay-Fit study were (1) age between 6 and 19 years, (2) having a chronic disease or physical disability including cardiovascular, musculoskeletal, metabolic, or neuromuscular disorders, (3) practiced sports less than twice a week during leisure time in the preceding 3 months or were advised to participate by their physical therapist or physician, (4) ability to understand simple commands, and (5) expected ability to perform the physical fitness tests. Youth using an electric wheelchair for sport purposes or those with progressive diseases were excluded. During the length of the study, children and adolescents were asked not to participate in other studies that could influence results in the current study. Parents and youth were invited by letter for participation in the Sport-2-Stay-Fit study.17 All parents and participants from 12 years of age provided written informed consent prior to study initiation.

Intervention

The HIT program was provided by an experienced physical educator and/or a physical therapist at school twice a week. This frequency is lower than the recommended training frequency for the general population (ie, 3 times a week).20 However, for youth with CP who may be deconditioned, it is recommended to start with 1 to 2 sessions of vigorous activity a week.21 Every training session consisted of a prescribed intensity, volume, and time (Table 1). The 30-second all-out approach was used, as recommended for interval training for youth,6,11,22 followed by active recovery of 90 or 120 seconds. The all-out approach means participants were extensively encouraged to sprint with maximal effort without need for extensive heart rate monitoring.10 Easily executable sprint exercises, like transferring bean bags and sprinting between cones, were chosen to provoke the cardiorespiratory system as much as possible within 30 seconds. For motivation, participants were instructed to count and report their repetitions to the trainers. Between intervals, participants performed lower intensive activity, such as gathering bean bags, as active recovery has been suggested to effectively aid the recovery process. The presence of participants was documented after every training session by the physical educator or physical therapist to measure adherence.

Outcome Measures

Anaerobic performance was the primary outcome, since daily physical activity in youth most often consists of intermittent bouts of intense exercise.11 Outcome measures were tested during school hours divided into 4 sessions both at baseline (T0) and within 2 weeks of the HIT (T1): (a) height, weight, anaerobic fitness, and strength, (b) aerobic fitness, (c) blood pressure, arterial stiffness, and body composition, and (d) metabolic profile. When possible, the same schedule was used for both T0 and T1 measurements. For analyses, participants were divided into 3 subgroups based on the Functional Mobility Scale (FMS): those who were able to run (ie, runners), those who could walk independently but were not able to run (ie, walkers), and those who propelled a manual wheelchair (ie, users of wheelchairs).23 A participant's FMS level was rated by the physical therapist or physical educator during free play or physical education.

Physical Fitness

Anaerobic Fitness. Both anaerobic performance and agility were assessed. Anaerobic performance was assessed by the Muscle Power Sprint Test (MPST).13,24 In this test children and adolescents had to complete 15-m sprints with a standardized rest of 10 seconds between sprints. Participants who could walk had to complete 6, 15-m runs, while users of wheelchairs completed 3, 15-m sprints at maximal pace. Power output was calculated using total mass (ie, body mass and wheelchair weight) and sprint times according to the following formula: power = (total mass × distance2)/(time3). Peak power was defined as the highest calculated power, while mean power was defined as average power over the 3 or 6 sprints. To assess agility, the 10 × 5-m sprint test was performed, either while running, walking, or propelling a wheelchair.13,24 During this test, the child had to sprint 10 times, as fast as possible, between 2 lines 5 m apart, while timed.

Aerobic Fitness. A shuttle run/ride test (SRT) was performed to assess both aerobic performance (achieved shuttles) and

O2peak. During this field test, a participant runs, walks, or rides between 2 markers, which were 10 m apart, as described extensively elsewhere.25,26 Participants had to adjust their running, walking, or wheelchair propulsion pace to the beep signals, until they failed to reach the line twice in a row, despite strong verbal encouragements. During the SRT a calibrated Cortex Metamax 3X (Samcon bvba, Melle, Belgium) measured oxygen uptake (

O2), carbon dioxide production (

CO2), peak heart rate, and respiratory exchange ratio (RER =

CO2/

O2) every 10 seconds, using metabolic stress test software (Metasoft Studio). To determine whether a subject reached maximal effort during the test, 2 of the following 3 criteria had to be achieved as reported previously in children and adolescents with CP: a heart rate of 180 bpm or more, an RER 1.00 or more at peak exercise, or subjective signs of intense effort, such as sweating, facial flushing, or a clear unwillingness to continue.27

Strength. To assess explosive strength, the standing broad jump or 1-stroke push was performed in participants who could walk and those propelling a manual wheelchair, respectively. The standing broad jump was assessed as the distance in centimeters jumped with 2 legs. The 1-stroke push was assessed as the distance in meters covered in the wheelchair with 1 push. Grip strength was measured using a hand-held dynamometer (CITEC, CIT Technics, Haren, the Netherlands). Participants sat on a chair or in their wheelchair with their elbows situated on the armrests. After a practice session, grip strength was performed 3 times with the preferred hand. Mean grip strength (N) was calculated by averaging the 3 attempts.

Cardiovascular Health

Blood Pressure. Both resting blood pressure and arterial stiffness were simultaneously and noninvasively measured with the Arteriograph (Litra, Amsterdam, the Netherlands). The measurement was performed in a supine position, after a 10-minute rest, using an inflatable cuff on the right upper arm. Participants were asked not to move or talk during the measurement.

Arterial Stiffness. Arterial stiffness was measured using 2 independent and validated techniques.28,29 The pulse wave velocity (PWV) was measured as the speed at which an aortic pulse travels according to the formula: PWV (m/s) = aortic length/(time/2), where aortic length was estimated with the jugulum-symphisis distance. Increased speed indicates stiffer arteries. The Augmentation Index (AIx) provides information on the peripheral resistance of the endothelial vessels according to the formula: AIx (%) = augmentation pressure/pulse pressure × 100. Increased index indicated higher peripheral resistance. Since AIx increases with age, z scores were calculated according to reference values of Hidvégi et al.28

Metabolic Health

Body Mass Index. Body mass index (BMI) was calculated as weight (kg)/height (m)². Weight was measured without shoes and orthoses on an electronic scale (Stimag, Hoofddorp, the Netherlands), which was also used to determine the weight of the participant's wheelchair. Height was either measured in standing or supine position depending on a child's walking level. In case a subject had legs with spasticity, arm span width was measured and corrected as described previously.30 To control for differences in age, z scores of BMI for age were calculated according to Dutch reference values.31

Body Composition. Fat mass was measured in the supine position using the Bodystat Quadscan 4000 (Euromedix, Leuven, Belgium) and determined with Bioelectrical Impedance Analysis. For waist and hip circumference, a horizontal measure was taken at the umbilicus and the greater trochanter, respectively.32

Metabolic Profile. To determine the metabolic profile, a finger puncture was performed from which total cholesterol, low-density lipoprotein (LDL), high-density lipoprotein (HDL), ratio (total cholesterol/HDL), triglyceride, and fasting glucose were determined. This was an optional measure and consent was collected separately. The finger puncture was performed using a Cholestech LDX analyzer (Mediphos Medical Supplies BV, Renkum, the Netherlands). All participants were instructed to refrain from eating and drinking 3 hours prior to this procedure. Before the finger puncture, participants were asked about the fasting period and, if possible, the measurement was postponed. Otherwise, only total cholesterol, LDL, HDL, and triglyceride data were used for analysis.33

Statistical Analysis

A sample size of 74 participants was calculated for the Sport-2-Stay-Fit study, of which the current study is a part, taking into account a dropout rate of 15%.17 For evaluating the effects of HIT on anaerobic performance, a sample size calculation was based on a previous exercise training study in children with CP where anaerobic performance was also measured with the MPST.3 Using a mean (standard deviation) training effect of 20.4 (38.0) W, with an α of 0.05 and power of 0.80, a sample size of 30 participants was required.

All data analyses were performed according to the intention-to-treat principle. We checked for normality of data, normality of residuals, and outliers. If residuals were not normally distributed, a log transformation was performed after which normal distribution was rechecked. Outcomes were normally distributed. The following outcomes were transformed before analysis: weight, anaerobic performance,

O2peak (L/min), HDL, and triglyceride. For comparison of outcomes pre- and posttraining, a paired sample t test was used to determine the effect of training. For the secondary analyses, a paired sample t test was used for the runners, walkers, and users of wheelchairs subgroups. Outcome measures were reported as means with standard deviations. If outcomes were changed significantly, the mean difference (MD) was reported. Effect sizes were calculated using Cohen d for significant differences. Values of Cohen d less than 0.20 were considered small, between 0.50 and 0.80 were considered medium, and effect sizes greater than 0.80 were considered large. All statistical analyses were performed using SPSS for Windows (version 21.0, SPSS Inc, Chicago, Illinois) with an α level of P = .05.

RESULTS

A total of 70 participants completed the study, but not all participants completed all assessments correctly, as depicted in Figure 1. Participant characteristics are shown in Table 2. The number of planned HIT sessions ranged from 9 to 16 in 8 weeks, due to the school schedule incorporating holidays, days off, and school activities with priority. The adherence was 84.5%, with an average number of 11.4 (3.0) training sessions. This was similar for all subgroups.

Aerobic Fitness. A total of 47 participants (67%) performed at their maximal effort both pre- and posttraining (Figure 1). There were no significant differences between pre- and posttest measurements for peak heart rate and RER (Table 3). Aerobic performance on the SRT improved in the total group (MD = 1.4 shuttles, P < .001, d = 0.94) and in all subgroups (Table 3 and Figure 2c). However, both absolute

Metabolic Health

In the total group, no changes were found in BMI, waist circumference, waist-hip ratio, and fat mass (Figure 2h), nor in any of the subgroups (Table 3), except for an increase in waist-hip ratio in the runners' subgroup (MD = 0.02, P = .037, d = 0.37). Eighty percent of participants provided consent but some (n = 11) had not fasted (Figure 1). No differences were found for the metabolic profile comprising total cholesterol, HDL, LDL, triglyceride, and glucose (Table 3).

DISCUSSION

No adverse events were reported during the study, suggesting HIT can be considered a safe and feasible intervention for this population. Similar results in anaerobic performance in healthy young adults were found following a 30-second all-out protocol 3 times a week for 6 weeks on a cycle ergometer.7 Improvements of 7% and 17% for mean and peak power were found compared with 9% and 10% in the current study, respectively. Another HIT program in overweight children consisted of 16 intervals of 15 seconds at 100% of maximal running speed followed by 15 seconds of active recovery.14 They found similar effects after 6 weeks; 11% improvement in aerobic performance on an SRT, compared with 11% in this study. In contrast, an 8-month training intervention composed of both aerobic and anaerobic exercise for 45 minutes twice a week resulted in improvements of 37% in aerobic performance on an SRT in children and adolescents with CP.3 Both the length of the program and lower anaerobic performance at baseline may explain the discrepancy.34 It is possible that our population was not deconditioned at baseline and therefore the current training could have been too low to induce positive effects other than exercise performance.

When comparing our data to the smallest detectable difference (SDD) reported in youth with CP who walk (SDD = 25 W), the improvements of anaerobic performance are clinically relevant for runners, but not for walkers.13 However, interpreting the improvements with the SDD for youth with CP must be done with caution, since our population comprised youth with disabilities in addition to CP. Furthermore, finding no improvement on aerobic performance in users of wheelchairs may be a result of a type II error, and should be re-examined in a larger sample of users of wheelchairs.

For

O2peak, a recent study in adolescents with CP reported a significant improvement of 10% in

O2peak (mL/kg/min) after 24 HIT sessions on a treadmill.16 They used an individualized protocol of 2 to 4 training sessions a week containing 1.5- to 4-minute interval resulting in 16 minutes at more than 85% of peak heart rate in total, with active breaks of about 70% of peak heart rate. However, these results were based on only 8 children with CP. In youth, approximately 40% of energy production during a 30-second sprint comes from aerobic metabolism.35 In addition, these all-out intervals cause endurance-like adaptations such as increased mitochondrial capacity, reduced lactate production during matched-work exercise, and increased lipid oxidation.6 However, since literature on 30-second all-out protocols on

O2peak in youth with physical disabilities is lacking, the effects on

O2peak remain unclear. For improving

O2peak with HIT in adults who are healthy, a recent meta-analysis suggests longer intervals (ie, 3-5 minutes) and a training frequency of 3 times a week.36 To improve

O2peak within 8 weeks in this population, we suggest future research to increase training frequency to 3 times a week and/or increase the time per interval (ie, training volume).

In the current study, baseline levels of resting systolic blood pressure were slightly elevated compared with nonoverweight reference values in youth who are developing typically.29 For children 13 years of age, reference values for systolic blood pressure for boys are 117 mm Hg and for girls 115 mm Hg compared with an average of 123 mm Hg in the current study. Although both systolic (MD = −2.9 mm Hg) and diastolic (MD = −2.4 mm Hg) blood pressure decreased after HIT, the improvement is small and the clinical relevance can be argued. Furthermore, the decrease in blood pressure was inconsistent across the subgroups. These findings are in line with a recent review showing inconsistent effects of HIT on blood pressure in adults with common metabolic diseases.10

This is the first study that measured the effect of HIT on arterial stiffness in youth with physical disabilities. Arterial stiffness, which is regarded to be the first sign of arteriosclerotic development, was within normal ranges at baseline for the total group in the present study.28,29 This was also shown in adults with CP in a study by McPhee et al.37 The normal values at baseline presumably explain why there was no reduction after training, similar to previous findings in other populations.38 However, in users of wheelchairs, AIx z scores were slightly elevated at baseline. In this subgroup an improvement can be seen in AIx; however, this finding needs to be confirmed in a larger sample. Our findings suggest that children and adolescents with physical disabilities at risk for arterial stiffness at baseline seem to benefit from HIT.

Recent studies, published after our design paper,17 have shown that positive effects of HIT on body composition occur either when long-term (>12 weeks) HIT is performed, and/or when participants are overweight at baseline.9,14,15 This may explain why we found no effects on body composition. Similar results were found after HIT in children with CP; no effects on BMI and fat mass were shown.16 The negative effect on waist-hip ratio in the runners' subgroup can probably be explained by a measurement error, since the hip circumference decreased unexpectedly. Although some studies found improvements in the metabolic profile after HIT, especially on HDL, triglycerides, and fasting glucose, evidence is inconsistent.9 A recent meta-analysis showed a positive but small effect on fasting glucose in overweight or obese adults, but not in the population who are normal weight.9 Our results are in accordance with recent findings showing that there is no evidence for improvement of the metabolic profile after a short-term HIT program, when baseline values are within normal ranges.9

There are several limitations in our study. Due to a small sample size (n = 9), results of the users of wheelchairs' subgroup should be considered exploratory. Since a control group with random assignment was lacking, current results do not account for natural development or other confounders. Outcome measures were not randomly tested across participants. The sample had a large age range, a variety of diagnoses, and mobility levels with consequently different methods between subgroups. This was however a deliberate choice, aimed to increase our sample size, and with the knowledge that many disabilities reported in the literature, such as CP, are often heterogeneous. Furthermore, this heterogeneity reflects real life as youth with physical disabilities probably exercise together at the schools at which they are recruited. In addition, the total number of sessions conducted within 8 weeks of HIT may not have been sufficient to elicit improvements but reflects daily life. Not all participants fasted for 3 hours prior to the cardiometabolic health measures. There was a dropout rate of 33% in aerobic fitness; however, no dropouts were the result of adverse events. A study in children with CP reported a dropout rate of 44% in 2 maximal exercise tests using similar criteria for maximal effort.27

For clinical practice, 30-second HIT is a time-efficient, feasible, and safe training in youth with physical disabilities who run, walk, or propel a manual wheelchair and is useful in improving some components of exercise performance in 8 weeks. For users of wheelchairs, evidence is limited and restricted to youth who propel a manual wheelchair. The short-term sprints are a good reflection of youth's daily life activity pattern, which is confirmed by the follow-up period of this study. After following the regular curriculum again for 6 months, the control group maintained its gains on anaerobic performance.39 This sustainability suggests that youth rehearsed and maintained their sprint capacities in their current activity pattern.

CONCLUSIONS

Following 8 weeks of HIT, anaerobic performance improved in youth with physical disabilities. In addition, youth with physical disabilities improved aerobic performance, with no changes in

O2peak. No effects were found for cardiometabolic health, except for a decrease in blood pressure. Training 3 times a week for at least 12 weeks is recommended to increase the effect on outcome measures. Since 87% of participants were either runners or walkers, evidence for users of wheelchairs is limited. Future research should therefore focus on users of wheelchairs. To improve

O2peak short-term, we suggest that future research increase training frequency to 3 times a week and/or increase time per interval (ie, training volume).

ACKNOWLEDGMENTS

We would like to thank the physical therapists and physical educators of all participating schools for special education in the Netherlands: Ariane de Ranitz, De Hoogstraat Rehabilitation in Utrecht, Lichtenbeek in Arnhem, Mariendael in Arnhem, and Heliomare in Wijk aan Zee. In addition, we would like to express our gratitude for the enormous amount of time and effort the participants, their parents, and students have put in, in order to make this study possible.